There are known hybrid electric vehicles which run on a combination of the power of an internal combustion engine and the power of an electric motor to thereby exhibit a good fuel economy (refer to, for example, patent literatures 1, 2). Among those hybrid electric vehicles, in hybrid electric vehicles referred to as a plug-in hybrid electric vehicle or an range extender EV which has an EV drive mode in which the vehicle can run only on the power of an electric motor, in order to make the fuel economy compatible with the driving performance of the vehicle, it is proposed to increase an area on a driving force curve where the electric motor can assist in the EV drive mode. Namely, if the EV drive mode in which the vehicle runs on the driving force of the electric motor can cope with a various types of driving from a low speed and high torque driving performed, for example, when the vehicle starts from a standstill to a high speed and low torque driving performed, for example, when the vehicle cruises at high speeds, the utilization of the internal combustion engine can be suppressed to as low a level as possible, which can improve the fuel economy remarkably. This is not limited to hybrid electric vehicles, and in case the area on the driving force curve where the electric motor can assist in the EV mode is also increased on electric vehicles, the driving performance thereof can be improved.